| Scope and method of study. The objective of this study is to evaluate whether minimum in-situ stresses in drilled earth formations can be obtained from data collected during normal drilling operations. Mathematical models of drilling penetration rate are inverted to solve for formation compressive strength. Knowledge of formation permeability or impermeability is used to relate compressive strength to effective confining pressure for each foot of drilled formation. Mohr's failure theory is used with the rock compressive strength to determine an angle of internal friction, which in turn is employed to calculate a "coefficient for earth at rest". This coefficient, together with known overburden, or vertical stress, and pore pressure, can be used to calculate an upper bound on the minimum principal horizontal stress for each foot drilled. The calculated in-situ stress bound profiles are compared with experimental field closure stress data obtained by Gas Research Institute contractors for four SFE (Staged Field Experiment) wells.; Findings and conclusions. The upper stress bound calculated by assuming formations are impermeable lies above all stresses obtained in field stress tests. A somewhat lower upper bound that falls closer to the measured stresses can be obtained by assuming that formations containing 90% or more sand are permeable, and recalculating the upper bound accordingly in these regions. Good results are obtained in SFE wells 1, 2 and 3, in east Texas, where tectonicly induced stresses are known to be absent. For SFE #4 in Wyoming, where tectonic stresses are active, an additive correction is needed to the calculated stress bound in order to give acceptable results when compared with experimental data. This correction is consistent with other approaches suggested in the literature to account for tectonic stresses. It is concluded that the proposed approach can be used to predict upper bounds on minimum principal in-situ stresses. |
